BERKELEY, Calif. (AP) — The University of California at Berkeley campus remained evacuated after a power outage cancelled evening classes and an explosion that led to one person being hospitalized for minor burns.

The explosion occurred in an underground utility vault outside California Hall as crews were trying to restore power on campus shortly before 7 p.m. Monday. In addition to the one person hospitalized, three others also received minor burns but refused treatment, said UC Berkeley spokesman Dan Mogulof.

“The explosion was about two stories high and as wide as a two-lane street,” Mogulof said. “We’re very fortunate there were no serious injuries.”

The blast may have been caused by an underground copper wire theft at a campus substation last week, Mogulof said. Officials thought they had repaired the damage Sunday.

With many buildings in the dark, fire crews also freed about 20 people stuck in dormitory elevators as a result of the outage that also led to the mandatory evacuation on campus.

It’s still unclear whether classes will resume on Tuesday.

“I have midterms tomorrow as do a bunch of other students on campus. A lot of students don’t have power to study or get their laptops on or anything like that,” Kelsie Carlson, a freshman, told KTVU-TV.

The power outage was reported around 4:30 p.m. Monday and led to evening classes being canceled as students were told to suspend all lab activities, Mogulof said.

About a half-hour later, authorities reported an apparent ammonia leak in a room at Latimer Hall. It was contained almost immediately, campus officials said.

That was followed by the explosion, Berkeley acting fire Chief Gil Dong said.

Campus officials said late Monday that they were slowly trying to restore power across campus in stages as campus e-mail was restored and other systems were coming online.